The present invention relates to methods and apparatus for perforating tubing inside a subsurface well, and more specifically to previously unknown manually-dropped perforating assemblies for use in perforating tubing inside a well.
Drilling wells has been fairly standard, that is, a hole is drilled and casing is installed and tubing is placed inside the casing to convey the production to the surface. The uses of perforators in the drilling of wells and in the servicing of wells already in production are manyfold.
Certain types of perforators are used to perforate holes in casing in order to start production. Such perforators are lowered to the firing point using one of three methods: lowered by an electric wireline, lowered by a slickline, or attached to the end of tubing and lowered through the casing. The latter is generally referred to as a tubing-conveyed perforator or gun. Perforators of casing are fired by an electrical firing system; by use of a drop bar, commonly referred to as a "go devil" system; or some type of hydraulic system. The expended perforator is then returned to the surface using one of the following methods: by an electric wireline or a slickline, or pulled to the surface along with the tubing string. And, in some cases, the perforator is released into the bottom of the well and left in an area of the wellbore below the production zone called the rat hole.
Occasionally, casing perforators are used to create a borehole at a precise depth in only the tubing in order to enhance production. Only rarely are such perforators used to perforate both tubing and casing.
Another use of perforators, other than in drilling operations, is in the servicing of producing wells. Frequently, it is necessary to perforate the tubing, within a well casing, so that trapped fluids can drain and escape from the tubing prior to the pulling of the tubing string from a well. Wells with plugged tubing require the tubing string to be pulled from the well. Therefore, inventors created perforating guns designed to make drain holes in the tubing, but not the casing, through use of a controlled explosion. Explosive perforating guns such as that disclosed in U.S. Pat. No. 4,624,307 to Kinley et al (1986) have been used to penetrate tubing.
Prior to this present invention, all known tubing perforators used for creating drain holes in tubing have been lowered into firing position by means of a wireline; either an electric wireline or a slickline. After detonation, such perforators have to be retrieved from the well before the tubing string can be pulled from the well. It is only after the tubing is pulled that the necessary steps can be taken to get the well back into production.
It is also possible to pull tubing from a well without draining the fluids from the tubing. And, pulling tubing without a drain hole is less costly than using a perforator which must be lowered by wireline. But, several problems occur when tubing is pulled without a drain hole. The time required to pull tubing out of a hole is increased due to the problem of trying to contain the fluids trapped in the tubing. A bucket designed to wrap around the connections of the tubing can be used. A hose is attached to the bottom of the bucket at one end with the other end attached to a tank which is mounted on a truck. When a tubing string section is unscrewed from another section, the fluids trapped inside flow into the tank. Some of the fluids invariably spill onto the surface. Clean up of such spillage is mandated by state regulation, and if contamination occurs outside the location of the well, clean up is mandated by the Environmental Protection Agency.
This present invention provides a perforating assembly which is less costly than wireline perforators and is even less costly to use than pulling tubing without a drain hole.
This invention provides a new perforating assembly which is manually dropped into well tubing from the surface. The firing mechanism of this tubing perforator has a frangible pin which is sheared when a detonating bar is dropped down the tubing. The resulting detonation perforates the tubing creating a drain hole. The design and method of use of this manually-dropped perforator permits the removal of the tubing string without the need to retrieve it in a separate step.
Heretofore known tubing perforators suffer from a number of disadvantages which result in a loss of production time. The following describes these disadvantages and provides the reasons the manually-dropped perforator is less costly to use:
(a) The hiring and rigging up of a wireline truck requiring two people, one wireline operator and one helper, is needed to use heretofore known perforators. Rigging the truck for the use of a wireline perforator is a step which is not required when the manually-dropped perforator is used.
(b) Wire slows the fall of a wireline perforator because of the drag caused by friction on the wire traveling through the well fluids and because of the drag of the wire on the inside of the tubing. So, the wire is lowered at a slower rate than the rate the manually-dropped perforator will fall.
(c) A wireline perforator may become stuck because its outside diameter is relatively larger than the diameter of the manually-dropped perforator. This can occur when the tubing is not fully open from the surface to the area where the tubing is plugged or when there are tight places in the tubing. The time-consuming process of retrieving the wireline perforator and lowering a second perforator is required when the first one does not reach the appropriate detonation point in the tubing string. The manually-dropped perforator is small enough to fall freely even if the tubing is not fully open.
(d) Another disadvantage of a wireline perforator is the problem of downhole pressure which can occur in the tubing when the wireline perforator is detonated. Fluid pressure between the casing and the tubing can be greater than the pressure inside the tubing. When a wireline-conveyed perforator is used to create a drain hole in the tubing, the release of this greater fluid pressure into the tubing drives the wireline-conveyed tool and the wireline itself upward. This causes tangled wireline and lost tools. The wireline and the tools almost always are left in the well. This problem simply does not exist with the manually-dropped perforator as there is no wire involved.
(e) Wireline perforators which use solid bullets, such as that disclosed in U.S. Pat. No. 4,624,307 to Kinley et al (1986) can become lodged in the tubing. This occurs when the bullets do not fully penetrate the tubing when fired, thus causing such perforator to become stuck in the tubing. In such cases, the wire must be cut and a second wireline perforator lowered into the well. This is a time-consuming process. The reliability due to the method of use of a Jet charge in the manually-dropped perforator avoids this problem.
(f) A second perforator may need to be lowered when the firing head on a wireline perforator malfunctions, a problem which occurs because the firing heads of heretofore known perforators are comprised of several moving parts. This problem is alleviated when the manually-dropped perforator is used because it is designed with only one moving part.
(g) Following perforation of tubing with the use of a wireline perforator, it is not possible to immediately start pulling the tubing from the well. This is because the wire and the perforator must be pulled out of the well before the tubing string can be pulled out. With the manually-dropped perforator, no wire is involved and the perforator does not need to be retrieved in a separate step, as it is raised with the tubing when the tubing is pulled out of the hole.
(h) With the use of an electrically detonated wireline perforator, workers can be injured when premature firing occurs due to electrical interference, such as radio waves. This invention is mechanically detonated, so such premature firing cannot occur.